Queue

Circular queues, zigzag iterators, FIFO matching with cancellation, and the data structure combinations that design problems keep using

LeetCode 622 note: a fixed array plus head and count, every position taken modulo the capacity, and count deciding empty versus full

LeetCode 1425 note: dp[i] = nums[i] + max(0, max of the previous k dp values); that max is a sliding window maximum, available in O(1) from a monotonic deque

LeetCode 281 note: a queue of unfinished sources, take one value in turn and put the source back if anything remains; it generalizes to k sources

LeetCode 3835 note: cost = (max - min) × length only grows as the window grows, so a sliding window counts subarrays; two monotonic deques keep the window’s max and min

LeetCode 3829 note: one queue each for riders and drivers; cancellations only update a set, and canceled riders at the front are dropped before matching

LeetCode 1696 note: dp[i] = nums[i] + max of the previous k dp values, with that max kept by a monotonic deque in O(n)

LeetCode 3508 note: a queue for FIFO, a set for duplicates, and an increasing time list per destination queried with bisect

LeetCode 387 note: count first, then scan left to right for the first character with count 1

LeetCode 232 note: two stacks, where in receives pushes and out is refilled from in only when empty, amortized O(1)

LeetCode 225 note: one queue suffices: after a push, rotate all earlier elements to the back so the new element sits at the front